1.4 Phototropism and polarotropism

1.4 Phototropism and polarotropism
The direction of protonemal growth is controlled by light. Protonemal cells grow toward a red light source (phototropism) or perpendicular toavibration plane of polarized red light, following a phenomenon known as ‘‘polarotropism” (B¨and 1958; Etzold, 1965). Phototropism  and polarotropism are phenomenologically different responses because in phototropism a protonema grows toward a light source whereas in polarotropism a
protonema grows perpendicular to the polarized incident light and to its vibration plane, regardless of the direction of incident light . Thus, the protonema grows toward the side that absorbs more light. Polarotropism occurs because of the orderly intracellular arrangement of phytochrome molecules attached to the plasma membrane. However the two light-induced tropisms may be considered equivalent if the direction of growth is determined by the highest
concentration of the far-red light absorbing form of phytochrome (Pfr) in the protonemal apical dome (explained in the following section). Hence, here I treat these two responses as one physiological phenomenon controlled by the same phytochrome molecular species and the same mechanism. When polarotropism is induced in protonemata growing on the surface of an agar medium, growth is not only perpendicular to the vibration plane but also toward the light source of the polarized light. In order to avoid phototropism under polarized light, to
irradiate cells evenly without reflection and refraction, and to eliminate the lens effect caused when cylindrical protonemal cells are elevated above the growing surface, all experiments were performed using protonemata cultured on an agar medium covered with a cover slip, or under similar submerged conditions.

When the direction of incident red light is changed, a change in the direction of protonemal growth toward the new light source can be detected about 1 hour after the light treatment. However, intracellular events show that the cells respond instantaneously to the new light by modifying the cytoskeletal pattern and subsequently the pattern of microfibrils (Wada et al., 1990). Analytical studies of the response have focused on either polarotropism or microbeam-induced phototropism, because the responses induced by these methods can be
controlled more accurately. The tropistic curvature of protonemata is very sharp when induced through whole cell irradiation by polarized light or by partial cell irradiation with a microbeam , but is more rounded when the whole cell is irradiated with ordinary light. It is likely that in the former instances light absorption by phytochrome molecules is restricted to a very small area compared to that in the latter.

Etzold (1965)proposed a hypothesis to explain the polarotropic response in Dryopteris protonemata. According to this hypothesis the red light absorbing form of phytochrome (Pr) is localized at the cell periphery (close to the plasma membrane) and has a transition moment parallel to the plasma membrane. Growth occurs in the portion of the cell where the highest concentration of phytochrome is transformed to the far-red light absorbing form (Pfr) by red light absorption . Based on this hypothesis, when polarized red light vibrating perpendicular to the cell axis is applied to the apical dome of protonemata, theprotonemata grow straight as before at their tip, because the phytochromes with transition moments parallel to the vibration plane of the polarized light are localized only at the tip of the apical dome. When the vibration plane is twisted (no longer perpendicular to the cell axis), a portion of the apical dome whose plasma membrane becomes parallel to the direction of the new vibration
plane can absorb more polarized light, and can then become a new growing cell tip.

According to Etzold’s hypothesis the transition moment of Pfr should be perpendicular to the plasma membrane (Etzold, 1965). The photo-conversion of the transition moment between Pr parallel and Pfr perpendicular to the plasma membrane by red and far-red light irradiation respectively, was confirmed through chloroplast movement in Mougeotia by Haupt and his colleagues (Haupt et al., 1969). In A. capillus-veneris we determined the actual photoreceptive
site in a protonema by microbeam irradiation with polarized red light at various portions of the cell and found that the cell margin, especially at the basal part of the apical dome, is the most effective site for polarized light absorption (Wada et al., 1981). The dichroic orientation of phytochrome was also confirmed in the polarotropism of A. capillus-veneris protonemata by very precise analyses (Kadota et al., 1982, 1985;Wada et al., 1983). Recently neochrome1
(neo1) (formerly called phytochrome 3 (phy3)) was discovered in A. capillus-veneris

Phototropism and polarotropism can be induced by blue light also, although, because blue light inhibits protonemal cell growth, it is better to irradiate with red light simultaneously to stimulate cell growth (Kadota et al., 1979, 1989). In these experiments, blue light was applied unilaterally or as polarized light, but red light was applied vertically in the former case and as non-polarized light in the latter case to avoid the directional influence of red light. The blue
light receptor of this phenomenon is not yet known, although phototropins are plausible candidates.
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